This invention relates to an on-line instrument which simultaneously measures the viscosity, density, and surface tension of fluids in their operating environment. It is based upon the principle of a damped, one-dimensional harmonic oscillator, and is specifically adapted for use when measuring the physical properties of fluids comprising a gas dissolved in a liquid. In particular, the instrument of this invention will accurately and simultaneously measure the operating density, viscosity, and surface tension of such fluids.
Knowing the physical properties of fluids, such as density, viscosity (as either the dynamic or kinematic viscosity), and surface tension, is essential when designing fluid handling equipment. Yet these properties vary to a considerable degree depending upon the precise operating conditions which the fluid encounters. In particular, the successful design and operation of refinery processing units depend on a knowledge of the physical properties of the process fluids at operating conditions. It is frequently difficult to obtain such data. Thus, physical properties, such as viscosity, density, and surface tension, are extrapolated from measurements made at much less severe conditions. Such extrapolations, however, interject error into design and operating calculations and create deficiencies in the refinery processing units.
For example, inaccurately calculated physical properties can lead to nonuniform temperature and flow distributions throughout fixed-bed reactors. This creates hot spots in the reactor and results in reduced product yields. To alleviate this situation, an instrument is needed that can measure physical properties up to 800.degree. F. and 3000 psia. Further, such an instrument must be capable of reproducing the precise environment encountered by the fluid in order to provide accurate measurements.
Fluids which comprise a gas dissolved in a liquid present an additional burden. The physical properties of these fluids depend upon the concentration of the gas in the liquid, which in turn depends upon the operating environment encountered at the time the fluid is formed. Removing the fluid from that environment may alter its physical properties. Of particular importance is the measure of viscosity.
Conventional instruments measure viscosity as either the ratio of an applied stress, such as gravity, to the resulting velocity, or, as the time required for a given volume of fluid to flow through a capillary or restriction. When viscosity is measured as the ratio of applied stress to resulting velocity it is referred to as absolute or dynamic viscosity and is measured in units called "poise" with the dimensions dyne-sec/cm.sup.2. When viscosity is measured as the flow rate of the fluid, it is referred to as kinematic viscosity. Kinematic viscosity is also the ratio of the dynamic viscosity to the density of the fluid and in the cgs system is measured in units called "stokes" with the dimensions cm.sup.2 /sec.
Viscometers generally fall into fundamental groups depending upon the measurement they take: those which measure kinematic viscosity, such as capillary viscometers; and those which measure dynamic viscosity, such as rotational viscometers, falling body viscometers, and oscillational viscometers.
Oscillational viscometers offer a number of desirable features and are useful over a wide range of viscosities. They are based on the relationship between the amplitude of the oscillations of an object oscillating in the fluid and the viscosity of the fluid. This relationship imposes restrictive experimental conditions on the use of oscillational viscometers so that they are not generally suited for use at elevated temperatures and pressures. Sandia National Laboratories ("Sandia") has, however, reported the development of an improved one-dimensional oscillational viscometer which overcomes certain of the restrictions encountered when measuring viscosity at elevated temperatures or pressures by measuring the rate of damping of an oscillating immersed plate suspended from a helical spring [Sandia National Laboratories (SAND 80-8034) Unlimited Release Printed October 1980, "A Single Apparatus For the Precise Measurement of the Physical Properties of Liquids at Elevated Temperature and Pressure", D. A. Nissen].
Despite advantages, the instrument reported by Sandia and similar instruments do not overcome the problems encountered when it is desirable to know the physical properties of fluids comprising a gas dissolved in a liquid. In order to employ conventional off-line oscillational viscometers, a fluid sample is placed in the instrument, and equilibrated at elevated temperature and pressure. Since the sample is brought to its operating conditions in the instrument outside of its operating environment, if the fluid comprises a gas dissolved in a liquid, the fluid sample may not represent the fluid in its operating environment. If the fluid sample is not representative of the fluid in its operating environment, conventional off-line instruments are ineffective.
Accordingly, an instrument which can be used to determine the physical properties such as density, viscosity, and surface tension of fluids comprising a gas dissolved in a liquid at elevated temperatures and pressures would be advantageous. It is the principle object of this invention to provide such an instrument.
Furthermore, instruments such as the one reported by Sandia are incapable of simultaneous measurement of viscosity, density, and surface tension. For example, Sandia's instrument is incapable of measuring viscosity and density with a single float. A thin plate is used to measure viscosity and surface tension but a larger volume bob is required to determine density. The change in equipment interjects error into the measurement and requires two separate samples, resulting in further possibility of error.
It is, therefore, highly desirable to have an instrument that can simultaneously measure viscosity, density, and surface tension, at elevated temperatures and pressures. It is a further object of this invention to provide such an instrument.